Method and apparatus for reverse circulating nuclear steam generator secondary fluid

ABSTRACT

A method and apparatus is disclosed for inducing reverse circulation of the fluid on the secondary side of a steam generator of the type including an outer vessel and a vapor generation chamber having an annular baffle means dividing the chamber into an outer annular downcomer passage and an inner riser section. A quantity of gas is introduced into the lower end of the annular downcomer passage and allowed to bubble up the downcomer passage, thus creating a region of fluid density in the downcomer less than the fluid density in the inner riser section. The density differential therebetween induces a natural circulation of the fluid down the riser section and up through the downcomer passage.

United States Patent 11 1 Stiteler et a1.

[ 1 Aug. 19, 1975 METHOD AND APPARATUS FOR REVERSE CIRCULATING NUCLEAR STEAM GENERATOR SECONDARY FLUID [75] Inventors: Fred Zwald Stiteler, New Hartford;

Ronald Louis Honigmann, Ellington, both of Conn.

[73] Assignee: Combustion Engineering, Inc.,

Windsor, Conn.

22 Filed: Apr. 19, 1974 21 Appl. No: 462,270

[56] References Cited UNITED STATES PATENTS 12/1963 Sprague 122 34 4/1966 Loebel 122/379 x Bergstrom .1 122/379 X Webster 122/32 57 ABSTRACT A method and apparatus is disclosed for inducing reverse circulation of the fluid on the secondary side of a steam generator of the type including an outer vessel and a vapor generation chamber having an annular baffle means dividing the chamber into an outer annular downcomer passage and an inner riser section. A quantity of gas is introduced into the lower end of the annular downcomer passage and allowed to bubble up the downcomer passage, thus creating a region of fluid density in the downcomer less than the fluid density in the inner riser section. The density differential therebetween induces a natural circulation of the fluid down the riser section and up through the downcomer passage.

10 Claims, 3 Drawing Figures PATENTED AUG 1 91975 SE-IZU 2 OF 2 FIG. 3

METHOD AND APPARATUS FOR REVERSE CIRCULATING NUCLEAR STEAM GENERATOR SECONDARY FLUID BACKGROUND OF THE INVENTION It is a desirable feature for the steam generators used in conjunction with nuclear steam supply systems to be capable of reverse circulating the fluid in the secondary side of the steam generator under certain circumstances. One very important application in which reverse circulation of the secondary fluid is required is when performing chemcial cleaning of the steam generator in order to eliminate crud deposits on the secon dary side of the tube bundle and tube sheets. Such a process involves filling the steam generator with the cleaning solution, and then reverse circulating the solution down the tube bundle and up the annular downcomer passage. Previous methods for circulating the chemical cleaning agent have required the use of elaborate pumping systems. Such systems typically involve a once-through flow of the cleaning solution and thus require large supply and waste storage tank capacities, and as a result also require large quantities of the chemical cleaning solution. Also in order to install such systems in steam generators not originally equipped, additional penetrations of the steam generator vessel may be required. In addition, when preparing such a system for actual use, time consuming removal of the steam dryers and steam separators from the interior of the steam generator vessel is required.

A second need for steam generator secondary fluid reverse circulation is during periods of reactor shutdown. During such periods it is necessary to maintain circulation of the steam generator water in order to maintain chemistry specifications of the secondary fluid in order to minimize corrosion and other undesirable chemical reactions occurring in the steam generator during the shutdown period. Such an arrangement also requires a pumping system of relatively large capacity as well as the associated piping, controls and other related hardware.

SUMMARY OF THE INVENTION There is disclosed herein a method of inducing reverse circulation of steam generator secondary fluid which employs the principle of natural circulation due to a difference in fluid density. This method may be used in any steam generator of the type including an outer annular downcomer passage and an inner cylindrical riser section into which a bundle of inverted U- tubes whose ends are secured to a lower tube sheet extend upwardly thereinto. For simplicity the steam generator may be envisioned as a U-tube with one leg being the inner riser section which contains the tube bundle and the other leg the downcomer passage. According to the present invention the secondary fluid density in the annular downcomer is reduced by injecting nitrogen or other suitable gas into the downcomer annulus adjacent to the lower end of the annulus. The gas bubbling up the downcomer produces a void fraction or a fluid-gas mixture of lower density than that in the tube bundle. As a result, a fluid density differential is established between the inner riser section and the downcomer and a natural circulation of fluid is induced from the inner riser region upwardly through the annular downcomer passage. In one embodiment the gas is introduced into the downcomer annulus through an annular gas distribution pipe which includes a plurality of holes along its length from which the gas issues to provide a uniform gas injection around the entire annulus. Spent gas rising from the surface of the fluid in the steam generator is vented at the top of the steam generator, collected and recirculated. Such a technique may be used both to induce natural circulation of the existing secondary fluid to maintain chemical specifications, or may be used to induce circulation of a suitable cleaning fluid which has been placed into the steam generator in place of the secondary fluid in order to chemically clean the secondary surfaces.

The system of the disclosed invention thus exhibits numerous advantages over prior art techniques for accomplishing reverse circulation of steam generator secondary fluid. Among these advantages are the following: (1) The system is simple and does not require use of pumps to circulate steam generator fluid. (2) It minimizes the quantity of cleaning solution required for chemical cleaning. This in turn reduces radiological hazards, minimizes cleaning solution supply and waste storage tank capacity, and solution feed pump capacity required is also minimized. (3) The system may be built into new steam generator systems as a permanent feature without interfering with normal operational requirements of the steam generator. (4) The system can be installed on existing steam generators without the need for additional penetration of the steam generator vessel.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional elevation of a nuclear steam generator employing secondary fluid circulation apparatus according to the present invention;

FIG. 2 is a sectional view taken along line 22 of FIG. 1; and

FIG. 3 is a sectional elevation, similar to FIG. 1 on a reduced scale, of a nuclear steam generator showing an alternate means for introducing gas into the annular downcomer passage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawings there is shown a nuclear operated steam generator 10 comprising an upright, generally cylindrical vessel I2 having dome shaped upper and lower closure caps, 14 and 16 respectively. A tube sheet 18 is provided in the lower end of the vessel dividing its interior into a lower heating fluid chamber 20 and an upper vapor generation chamber 22. The heating fluid chamber 20 is separated into inlet and outlet portions, 24 and 26 respectively by an upstanding partition plate 28, and appropriate connections are provided for circulating heating fluid into and out of the vessel. The vapor generation chamber 22 contains an annular baffle means 30 dividing the chamber into an outer annular downcomer passage 32 and an inner riser section 34. A bundle of inverted U-tubes 36 whose ends are secured to the tube sheet 18 in communication with the inlet 24 and outlet 26 portions of the heating fluid chamber 20 is provided in the riser section 34 for passing heating fluid in heat exchange relation with vaporizable liquid that enters the vessel through an inlet 37. In normal operation the vaporizable liquid admitted to the vessel is circulated downwardly through the downcomer passage 32 and thence upwardly into the riser section 34, in the latter of which a portion thereof is transformed into vapor. The upper end of the riser section 34 is closed by a horizontally disposed baffle plate 38 which, together with baffle means 30, defines a vapor liquid mixture collection plenum 40. The baffle plate 38 is provided with a plu rality of openings (not shown) to which are attached upstanding connecting pipes 42 for mounting separator apparatus 44 in communication with the collection plenum 40. Overlying the separator apparatus 44 is a second baffle plate 46 extending across the full cross section of the cylindrical vessel 12, this plate is also provided with a plurality of openings to which are attached upstanding steam dryer modules 48. Steam generated in the riser section 34 passes upward through the steam separators 44 and dryers 48 to a vapor plenum 50 in the upper end of the vessel. A vapor outlet nozzle 52 communicates with the vapor plenum 50 for conducting the processed vapor through steam line 54 and shutoff valve 56 to a point of use.

Referring now to FIGS. 1 and 2, there is disposed in the annular downcomer passage 32 adjacent the lower end of the baffle means 30 an annular gas distribution pipe 58 which is positioned about midway between the outside of the baffle means 30 and the inner wall of the cylindrical vessel 12. This annular gas distribution pipe 58 is provided with a plurality of uniformly spaced gas openings 60 on the upper side thereof and also communicates with a gas supply pipe 62 which passes through an already existing handhole penetration 64 in the cylindrical pressure vessel 12. A gas vent 66 is provided in the upper closure cap 14 through which gas which has bubbled up through the downcomer annulus 32 may be vented from the vapor plenum 50 in the upper end of the cylindrical vessel 12. The gas thus vented is passed through recirculation line 68 and passes to a nitrogen storage tank 70 and from there is drawn as needed by nitrogen gas supply pump 72 which recirculates the gas via the gas introduction line 62 back to the gas distribution pipe 58 in the lower end of the downcomer annulus 32.

When it is desired to conduct the chemical cleaning operation of the secondary side of the steam generator the existing secondary fluid is first pumped or drained from the interior of the cylindrical vessel 12. The vessel is then filled with a suitable cleaning fluid which may be any of a number of commercially available acid solutions designed specifically for the cleaning of steam generators. The level of the cleaning fluid in the steam generator should be below the position of thenitrogen gas vent 66, and at least as high as the elevation of the baffle plate 38. However, if it is desired to clean the separators 44 and dryers 48 the fluid level should be above the upper level of the steam dryers 48 in order to insure flow of cleaning fluid through this apparatus. The steam valve 56 is shut off, and other penetrations are sealed to insure that the cleaning fluid is confined to the interior of the vessel 12. The nitrogen pump 72 is then activated and caused to introduce nitrogen gas into the gas distribution annular pipe 58 from which gas issues through openings 60 to provide a uniform gas injection around the entire downcomer annulus 32. The nitrogen gas thus introduced tends to bubble uniformly up through the downcomer annulus 32 to produce a void fraction or a fluid-gas mixture of lower density than that of the fluid in the tube bundle region 34. As the lower fluid density is established in the annular downcomer passage 32 a natural circulation of cleaning fluid is established due to the density differential between the tube bundle region 34 and the annular downcomer passage 32. The cleaning fluid thus flows downwardly through the tube bundle region 34, passes around the lower end of the baffle 30 and thence upwardly through the annular downcomer passage 32. The cleaning fluid flowing through the downcomer annular passage continues moving upward into the vapor plenum region 50 in the upper end of the vessel. The nitrogen gas contained in the cleaning fluid passes from the cleaning fluid in this region and is vented from the top of the steam generator through the gas discharge vent 66. The vented gas is then collected in nitrogen tank 70, and in turn recirculated back to the lower end of the downcomer passage 32 via nitrogen pump 72 and nitrogen introduction line 62. The cleaning fluid which has passed into the vapor chamber 50 then passes downwardly through the steam dryer apparatus 48 and through the steam separator apparatus 44 into vapor liquid mixture collection plenum 40 and from there passes down through the tube bundle region 34 and is again caused to recirculate upward through the downcomer passage following the same route as has just been described.

It has been found that in order to insure uniform flow and cleaning it is desirable that the cleaning fluid flow at a velocity of about 0.5 to l ft/sec through the tube bundle region of the steam generator. This velocity corresponds to a volume flow of approximately 33,000 gals/min (GPM). In order to induce a flow of this volume it is necessary to introduce approximately 1500 standard cu. ft./min of nitrogen gas through the annular gas distribution pipe 58. The entire cleaning operation is ideally carried out at a temperature of approximately 250F. While the invention has been described as using nitrogen gas as the circulation gas for the reason that nitrogen possesses the advantages that it is inert to any of the chemicals present in the secondary fluid, it serves to purge undesirable gases such as oxygen from the system and is also relatively inexpensive; other suitable gases could be used with equal success in achieving the circulation desired. For example, in some chemical cleaning applications it may be desirable to use oxygen in place of nitrogen to aid in oxidizing copper deposits.

Another application of the present invention is to induce a circulation of the existing secondary fluid in the steam generator during periods of shutdown in order to maintain uniformity of the chemistry and maintain chemistry specifications throughout the steam generator for purposes of reducing corrosion and other undesirable chemical reactions during the shutdown period. In order to achieve such circulation all that is necessary is that sufficient steam generator fluid be added to the steam generator vessel to bring the fluid level above the baffle plate 38. The nitrogen pump 72 is then actuated and introduces nitrogen gas into the annular gas distribution pipe 58 located adjacent to the lower end of the annular downcomer passage 32. Natural circulation is then induced in the same manner as described in connection with the cleaning operation, thus causing a circulation up through the downcomer passage down through the openings in the steam separator apparatus 44 and downward through the tube bundle region.

Referring now to FIG. 3, a second embodiment of the apparatus for introducing nitrogen gas into the lower end of the downcomer annulus 32 is shown. In this embodiment an annular nitrogen gas introduction header 74 is positioned near the upper end of the downcomer annulus 32 just underlying the region in which the steam generator fluid is introduced through inlet penetration 37. The nitrogen supply line 62 passes through a penetration 76 in the cylindrical vessel 12 and communicates with the distribution header 74. Extending downwardly from the annular distribution header 74 are a plurality of smaller diameter nitrogen supply lines 78, these lines extend downward through the annular downcomer passage 32 and terminate at a point just above the lower end of the downcomer baffle 30. The ends of each of these downwardly extending nitrogen distribution lines 78 is provided with a T-type fitting 80 which extends through a horizontal segment of the downcomer annulus. Each of the T-fittings 80 is provided with a plurality of nitrogen discharge openings (not shown) on the upper side thereof. Thus, in operation nitrogen gas is delivered to the upper annular nitrogen distribution header 74 and passes downward through lines 78 and delivered uniformly at the lower end of the annular downcomer passage through T- fittings 80, thus inducing natural circulation of the fluid in the steam generator as in the previously described embodiment. This second described embodiment will have particular application in a steam generator structure wherein the size of the downcomer annular passage 32 and the required rate of flow of secondary coolant fluid through the annular downcomer passage makes the positioning of an annular pipe in a region of narrow flow cross section undesirable as it would tend to increase the flow resistance of the secondary fluid in this region. This problem is overcome in the second embodiment by positioning the annular gas distribution header 74 in a region above the downcomer annulus which is substantially larger in cross section than at the lower end of the downcomer where the nitrogen gas is introduced.

It will be understood that various changes in the details, materials and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

What is claimed is:

1. A method of inducing fluid circulation on the secondary side of a steam generator of the tupe including an outer vessel and a vapor generation chamber having an annular baffle means dividing the chamber into an outer annular downcomer passage and an inner riser section, wherein the fluid level is said steam generator is above said annular baffle means, comprising the step of: introducing gas into the lower end of the annular downcomer passage, whereby said gas bubbles up the downcomer passage creating a region of fluid density in the downcomer passage less than the fluid density in the inner riser section, the density differential therebetween inducing a natural circulation of the fluid down the inner riser section and into and up through the downcomer passage.

2. A method according to claim 1 including venting out of the outer vessel gas which has risen through the annular downcomer and passed from the fluid into the region above the fluid level.

3. A method according to claim 2 including collect ing the gas which is vented from the outer vessel and re-introducing said gas into said annular downcomer passage in said introducing step.

4. A method according to claim 1 wherein said gas introduced into the lower end of the annular downcomer passage is nitrogen gas.

5. A method of cleaning the secondary side of a steam generator of the type including an outer vessel and a vapor generation chamber having a annular gaffle means dividing the chamber into an outer annular downcomer passage and an inner riser section comprising the steps of:

removing the normal secondary fluid from the secondary side: introducing sufficient fluid cleaning agent into the secondary side to bring the level of same to a level at least above said annular baffle plate; and

introducing gas into the lower end of the annular downcomer passage, whereby said gas bubbles up the downcommer passage creating a region of cleaning fluid density in the downcomer passage less than the cleaning fluid density in the inner riser section, the density differential therebetween inducing a natural circulation of cleaning fluid down the inner riser section and into and up through the downcomer passage.

6. Apparatus for inducing fluid circulation on the secondary side of a steam generator of the type including an outer vessel and a vapor generation chamber having an annular baffle means dividing the chamber into an outer annular downcomer passage and an inner riser section, wherein the fluid level is above said annular baffle means, comprising: means for introducing gas into the lower end of the annular downcomer passage, whereby said gas bubbles up the downcomer passage creating a region of fluid density in the annular downcomer passage less than the fluid density in the inner riser section, the density differential therebetween inducing a natural circulation of the fluid down the inner riser section and into and up through the downcomer passage.

7. Apparatus according to claim 6 wherein said means for introducing gas comprises and annular gas distribution pipe disposed within and at the lower end of said annular downcomer passage, said annular gas distribution pipe including a plurality of gas discharge openings on the upper side thereof; and gas supply means in flow communication with said annular gas distribution pipe.

8. Apparatus according to claim 6 including means for venting out of the outer vessel, gas which has risen through the annular downcomer and passed into the region above the fluid level.

9. Apparatus according to claim 8 including means for collecting the gas vented from the outer vessel by said venting means; and means for re-introducing said collected gas into said annular downcomer passage through said means for introducing gas.

10. Apparatus according to claim 6 wherein said annular downcomer passage has a region of larger cross sectional flow area adjacent the upper end thereof and wherein said means for introducing gas comprises: an annular gas distribution within said region of larger cross sectional flow area; and a plurality of gas distribution pipes extending vertically downward through said annular downcomer passage to a position adjacent the lower end thereof, each of said vertically extending gas discharge pipes including a gas discharge fitting at the lower end thereof, each of said fittings containing a plurality of gas discharge openings on the upper side thereof.

UNITED STATES PATENT OFFICE QETIFICATE OF QORRECTIUN PATENT NO. 3 9OO,OlO DATED August 19, 1975 |N 1 Fred Z. Stiteler et al It is certified that error appears in the ab0ve-identified patent and that said Letters Patent are hereby corrected as shown below:

Column Column Column Column Column [SEAL] 6, line 39 delete "and" and insert -an-.

6, line 59 following "distribution" insert -hea.der disposed-.

Erigncd and Sealed this twenty-seventh D ay Of April 1 9 76 A ttest.

RUTH C. MASON Arresting Officer C. MARSHALL DANN ('mrrmissr'unvr uj'lurents and Trademarks 

1. A method of inducing fluid circulation on the secondary side of a steam generator of the tupe including an outer vessel and a vapor generation chamber having an annular baffle means dividing the chamber into an outer annular downcomer passage and an inner riser section, wherein the fluid level is said steam generator is above said annular baffle means, comprising the step of: introducing gas into the lower end of the annular downcomer passage, whereby said gas bubbles up the downcomer passage creating a region of fluid density in the downcomer passage less than the fluid density in the inner riser section, the density differential therebetween inducing a natural circulation of the fluid down the inner riser section and into and up through the downcomer passage.
 2. A method according to claim 1 including venting out of the outer vessel gas which has risen through the annular downcomer and passed from the fluid into the region above the fluid level.
 3. A method according to claim 2 including collecting the gas which is vented from the outer vessel and re-introducing said gas into said annular downcomer passage in said introducing step.
 4. A method according to claim 1 wherein said gas introduced into the lower end of the annular downcomer passage is nitrogen gas.
 5. A method of cleaning the secondary side of a steam generator of the type including an outer vessel and a vapor generation chamber having a annular gaffle means dividing the chamber into an outer annular downcomer passage and an inner riser section comprising the steps of: removing the normal secondary fluid from the secondary side: introducing sufficient fluid cleaning agent into the secondary side to bring the level of same to a level at least above said annular baffle plate; and introducing gas into the lower end of the annular downcomer passage, whereby said gas bubbles up the downcommer passage creating a region of cleaning fluid density in the downcomer passage less than the cleaning fluid density in the inner riser section, the density differential therebetween inducing a natural circulation of cleaning fluid down the inner riser section and into and up through the downcomer passage.
 6. Apparatus for inducing fluid circulation on the secondary side of a steam generator of the type including an outer vessel and a vapor generation chamber having an annular baffle means dividing the chamber into an outer annular downcomer passage and an inner riser section, wherein the fluid level is above said annular baffle means, comprising: means for introducing gas into the lower end of the annular downcomer passage, whereby said gas bubbles up the downcomer passage creating a Region of fluid density in the annular downcomer passage less than the fluid density in the inner riser section, the density differential therebetween inducing a natural circulation of the fluid down the inner riser section and into and up through the downcomer passage.
 7. Apparatus according to claim 6 wherein said means for introducing gas comprises and annular gas distribution pipe disposed within and at the lower end of said annular downcomer passage, said annular gas distribution pipe including a plurality of gas discharge openings on the upper side thereof; and gas supply means in flow communication with said annular gas distribution pipe.
 8. Apparatus according to claim 6 including means for venting out of the outer vessel, gas which has risen through the annular downcomer and passed into the region above the fluid level.
 9. Apparatus according to claim 8 including means for collecting the gas vented from the outer vessel by said venting means; and means for re-introducing said collected gas into said annular downcomer passage through said means for introducing gas.
 10. Apparatus according to claim 6 wherein said annular downcomer passage has a region of larger cross sectional flow area adjacent the upper end thereof and wherein said means for introducing gas comprises: an annular gas distribution within said region of larger cross sectional flow area; and a plurality of gas distribution pipes extending vertically downward through said annular downcomer passage to a position adjacent the lower end thereof, each of said vertically extending gas discharge pipes including a gas discharge fitting at the lower end thereof, each of said fittings containing a plurality of gas discharge openings on the upper side thereof. 